Nacrite is the rarest of the kaolin polymorphs known. It is a 1:1 aluminous dioctahedral phyllosilicate (clay) mineral, and most occurrences are associated with hydrothermal or pneumatolytic processes. Like all kaolin minerals, each kaolin sheet has one silica tetrahedral layer and one aluminum octahedral layer; the crystalline structure was initially described as a 6-layer kaolin mineral with rotation of the empty octahedral sites to produce a spiral around the c axis. The c dimension was approximately 43 Angstroms. However, the crystal can be described as a monoclinic structure having two layers and a larger beta angle. The octahedral layer of one crystal and the tetrahedral layer of the one above or below. The crystal structure displayed to the left is based on crystal refinement derived from x-ray diffraction data.

Zheng, H., and S.W. Bailey. 1994. Refinement of the Nacrite Structure. Clays and Clay Minerals. 42(1):46-52.